Organic carbonates are useful as intermediates in numerous chemical processes and as synthetic lubricants, solvents, plasticizers and monomers for organic glass and various polymers, including polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transparency, shock resistance and processability.
One method for the production of polycarbonate resin employs phosgene and bisphenol-A as starting materials. However, this method has numerous drawbacks, including the production of corrosive by-products and safety concerns attributable to the use of the highly toxic phosgene. As such, polycarbonate manufacturers have developed non-phosgene methods for polycarbonate production, such as reacting dimethyl carbonate with bisphenol-A.
Dimethyl carbonate has a low toxicity and can be used to replace toxic intermediates, such as phosgene and dimethyl sulphate, in many reactions, such as the preparation of urethanes and isocyanates, the quaternization of amines and the methylation of phenol or naphthols. Moreover, it is not corrosive and it will not produce environmentally damaging by-products. Dimethyl carbonate is also a valuable commercial product finding utility as an organic solvent, an additive for fuels, and in the production of other alkyl and aryl carbonates.
Dimethyl carbonate, as well as other organic carbonates, have traditionally been produced by reacting alcohols with phosgene. These methods have the same problems as methods that use phosgene and bisphenol-A, i.e., the problems of handling phosgene and disposing of phosgene waste materials. Thus, there is a need for commercially viable non-phosgene methods for the production of dimethyl carbonate, as well as other organic carbonates.
Methods have been proposed for preparing dimethyl carbonate by the catalytic reaction of methanol with carbon monoxide and oxygen in accordance with the following equation: ##STR1##
The copper compounds acting as catalysts in such a reaction are in the form of various copper salts. However, there are problems associated with using many of the proposed copper salts in an industrial process. For example, the use of copper(II) chloride as a catalyst gives unsatisfactory selectivities. Moreover, problems are caused by the formation of relatively large amounts of methyl chloride, which, because of its high volatility, is difficult to contain and can lead to corrosion in virtually the entire production plant.
Although the use of other catalyst salts in the form of organic complexing agents generally provides better selectivities than salts such as copper(II) chloride, such agents are typically only partially dissolved in the reaction mixture and the undissolved catalyst salts can cause processing problems. Specifically, the undissolved salts have to be conveyed through the reaction zone and cooling equipment and thereafter separated by mechanical means, such as a centrifuge. Such a method results in corrosion, poor heat transfer and blockages and encrustations associated with the undissolved salts.
One method that avoids the circulation of undissolved catalyst salts through the plant uses an excess of methanol. The process retains the suspended catalyst salts in the reactor and meters the methanol, carbon monoxide and oxygen feeds to the reactor, while removing dimethyl carbonate, water of reaction and methanol by distillation. However, the use of excess methanol results in a relatively low reaction rate and a low concentration of dimethyl carbonate. Additionally, since the reaction is carried out at high pressures and the solubilities of both dimethyl carbonate and water in the reaction medium are very high, it is difficult to separate the dimethyl carbonate and water.
Other methods for producing dialkyl carbonates, which utilize a homogeneous catalyst system, have been proposed. One method reacts an alcohol with carbon monoxide and oxygen in the presence of a catalyst system composed of complexes of metals which are capable, by oxy-reduction, of displaying two valency states. Although such methods may be satisfactory in producing alkyl carbonates, their use on an industrial scale has some drawbacks. Beyond the relatively high cost of complex catalyst systems, these systems have a low conversion of the alcohol because they are sensitive to the water and carbon dioxide formed together with the carbonate in the course of the reaction. There are difficulties, moreover, in the separation of the reaction products and more particularly of water and carbonate from the reactor effluent and from the homogeneous catalyst inasmuch as the ligand is normally an organic base and brings about a certain hydrolysis of the carbonate due to the water which is present in the system.
Methods have also been proposed for producing dialkyl carbonates by reacting an alkanol, carbon monoxide and oxygen in the presence of a catalyst which is heterogeneous to the reaction mixture, such as reacting the alkanol with carbon monoxide and oxygen in the vapor phase in the presence of a zeolite catalyst containing copper; reacting an alkanol, with carbon monoxide and oxygen in the vapor phase, in the presence of a catalyst containing: (1) a copper halide, a copper oxyhalide, or a copper carboxylate halide, (2) a quaternary ammonium salt, and (3) a support component; and reacting an alkanol with carbon monoxide and oxygen, in the vapor phase in the presence of a catalyst having a nitrogen-containing coordination compound copper hydrocarbyloxy halide complex supported on activated carbon.
Although such methods may be commercially viable, there still exists a need for economical methods for producing organic carbonates, such as dimethyl carbonate, on an industrial scale, due to the market potential for such products.
Other processes proposed for producing organic carbonates without the use of phosgene are the reaction of urea or urethanes with alcohols in the presence of catalysts, the reaction of alkyl halides or sulphates with alkaline carbonates, the reaction of carbon monoxide with alkyl nitrites in the presence of catalysts, the reaction of alcohols with carbon dioxide and electrochemical synthesis. However, these processes have little practical importance for producing organic carbonates on an industrial scale.
Thus, there is a need for an economical method of producing organic carbonates, such as dimethyl carbonate, on a commercial scale which does not have the above mentioned disadvantages.